Functional connectivity of the human face network exhibits right hemispheric lateralization from infancy to adulthood

Adults typically exhibit right hemispheric dominance in the processing of faces. In this cross-sectional study, we investigated age-dependent changes in face processing lateralization from infancy to adulthood (1–48 years old; N = 194). We co-registered anatomical and resting state functional Magnetic Resonance Imaging (fMRI) scans of toddlers, children, adolescents, and adults into a common space and examined functional connectivity across the face, as well as place, and object-selective regions identified in adults. As expected, functional connectivity between core face-selective regions was stronger in the right compared to the left hemisphere in adults. Most importantly, the same lateralization was evident in all other age groups (infants, children, adolescents) and appeared only in face-selective regions, and not in place or object-selective regions. These findings suggest that the physiological development of face-selective brain areas may differ from that of object and place-selective areas. Specifically, the functional connectivity of the core-face selective regions exhibits rightward lateralization from infancy, years before these areas develop mature face-selective responses.


Results
Comparing visual and resting state experiments in adults from Rosenthal et al. 29 We performed a repeated measures ANOVA-within-subject factorsscan, network (as described in Functional connectivity analysis), and hemisphere.Face selective regions exhibited main effects for scan (F (1,19) = 386.353,p< 0.0000, η 2 p = 0.953), network (F (1,19) = 138.189,p< 0.0000, η 2 p = 0.879) and hemisphere (F (1,19) = 31.656,p< 0.0001, η 2 p = 0.625).The ANOVA also revealed an interaction between scan and network (F (7,133) = 15.518,p< 0.0000, η 2 p = 0.449), but no interaction was found between scan and hemisphere (F (1,19) = 2.491, p=0.131, η 2 p = 0.116).As expected, we found an interaction between hemisphere and network (F (7,133) = 11.017,p< 0.0000, η 2 p = 0.367).In addition, there was a 3-way interaction between scan, hemisphere, and network (F (7,133) = 2.976, p=0.006, η 2 p = 0.135).Therefore, we compared the connectivity of the right and left hemispheres in each network (see more Tables S4A and   S4B).The main networks we focused on, the core face regions, the place selective regions and the object selective regions, replicated the expected pattern during both the visual and the resting state experiment such that the within face connectivity in the core face ROIs exhibited right dominance.The connectivity within the place selective regions was greater in the left hemisphere in the visual experiment condition, while no hemispheric difference was found in the resting state condition.In the object selective regions, no difference was found between the hemispheres in both scanning conditions.Our focus was the hemispheric differences in the functional connectivity within the main visual networks (core face, place and object) and not in the extended face network and between the visual network, yet we included these data in our analysis (see Tables S4A and S4B).
Importantly, this pattern of results replicates the organization of the face selective regions expected from previous studies 13, 27 using standard analysis approaches and hence validates the new methodology developed in the present study.interaction between these factors (F (7, 413) = 5.842, p<0.00001, η 2 p =0.090).A main effect was found for hemisphere (F (1,59) = 45.744,p<0.000001, η 2 p =0.437) and no interaction between hemisphere and group (F (1,59) =0.005, p=0.944, η 2 p = 0).As expected, we found an interaction between network and hemisphere (F (7,413) =9.019, p<0.000001, η 2 p =0.133).There was no interaction between network, hemisphere, and group (F (7, 413) =1.246, p=0.276, η 2 p = 0.021).Hence, we compared the between connectivity in each hemisphere and network (Tables S5 A & B).As expected, and described above, in the adult group that participated in both the visual and resting state scans, the core face selective regions exhibited right dominance in both groups of adults and no difference in hemispheric connectivity within the connectivity of the place network in both groups.In the object selective regions, no hemispheric difference was found in the adult group from Rosenthal et al. 29 , while the adults from the ABIDE dataset exhibited right dominance.Yet, the effect sizes of the functional connectivity in core face selective regions were larger compared to the object selective regions in the group of ABIDE adults.In addition, object selective regions within each hemisphere were based only on one edge and hence might be unstable.These findings validate that it is possible to use the same approach for other groups for whom obtaining task-related data during scanning is difficult or even impossible such as toddlers.
To examine the hemispheric dominance of each network in each age group, for each network in each age group we compared the right and the left functional connectivity (see more in Tables S6A-E).The connectivity of the core-face selective regions was larger in the right compared to the left hemisphere in all age groups.While the within place connectivity exhibited no hemispheric differences in the other age groups.Within the object selective regions, all age groups exhibited no hemispheric difference except for the adults' group that showed larger connectivity in the right hemisphere.The tables below also include analysis of hemispheric differences in the functional connectivity within the visual networks and between the visual network and within the extended face network (see Tables S6A -E).
This pattern of results suggests that the core-face selective regions have an early right biased organization which is specific to faces as the organization differs from the organization of the place and object selective regions.To examine the hemispheric dominance of each network in each age group, for each network in each age group we compared the right and the left functional connectivity (see more in Tables S8A-C).The connectivity of the core-face selective regions was larger in the right compared to the left hemisphere in all age groups.No right dominance was found within the place connectivity and within the object connectivity exhibited in all age groups (children, adolescents, and adults).The tables below also include analysis of hemispheric differences in the functional connectivity within the visual networks and between the visual network and within the extended face network (see Tables S8A -C).

Time series signal-to-noise ratio (tSNR) analysis
To test whether greater functional connectivity in the right compared to the left core face network was not obtained due to differences in signal quality across the two hemispheres 46 , we evaluated the time series signal-to-noise ratio (tSNR) across hemispheres in this network in the different age group.For each subject, we measured the tSNR during the resting state scans in each ROI of the core face network.As per previous publications, tSNR was computed for each voxel as follows: tSNR = mean(time series)/SD(time series) 45 .
We first investigated the difference of the tSNR between the general tSNR in all the visual regions in the right hemisphere compared to the left hemisphere.Second, we investigated the tSNR within the core face regions in the right hemisphere compared to the left hemisphere.In both analyses we conducted a repeated measures ANOVA with the between factors: group (toddlers from Dinstein et al.A comparison between the general tSNR of the right hemisphere and the left hemisphere in each group, only revealed that late adolescents had larger tSNR in the left hemisphere compared to the right hemisphere. we examined which edges between specific nodes of the core face network contributed the most to the right hemispheric lateralization.To do so, we used a linear discrimination analysis (LDA) 47 .The age of each participant and the functional connectivity between the fROIS of the visual selective regions (core-face, places and objects) were defined as classifiers of the right and left hemisphere.This model exhibited 80.818% classification accuracy, while testing the prediction with Leave One Out Cross-Validation (LOOCV) exhibited an accuracy level of 78.302%.Table S10 describes which features donated most to this classification.Importantly, age had the lowest absolute value, suggesting that this feature shows the least contribution in predicting the hemispheric laterality.This further confirms our main result of right lateralization in all age groups.When examining the contribution of specific edges, we found that, within the core-face selective regions, the functional connectivity between the FFA and LOF, FFA and OFA, OFA and LOF, and LOF and pSTS had a positive and the highest absolute values.This suggests that these functional connections contributed the most to predicting the right hemispheric laterality.This result is consistent with the known role of these regions in face processing 10 .

Table S2 -
Instructions given to participants in each resting state experiment in each site:

Table S3 -
Number of voxels within each fROI on the anatomical template space:

Table S4A :
Visual experiment condition -Comparison of functional connectivity across hemispheres in each network.

Table S4B :
Resting state condition -Comparison of functional connectivity across hemispheres in each network.

Comparing resting state scans from Rosenthal et al. 29 and the ABIDE database Our
next goal was to use our newly developed methodology on subjects that could not perform a task during scanning and were only scanned during resting state and hence did not have their own ROI definition.Specifically, in the following analysis, we aimed to validate the usage of resting state data and we therefore compared between adults' resting state from the previous stage (see Supplementary Information -Comparing visual and resting state experiments in adults from Rosenthal et al.29and adults from the ABIDE database.We performed a repeated measures ANOVA between factors: group (Rosenthal et al. 29/ ABIDE), within factors: network (as described in Functional connectivity analysis) and hemisphere.Similarly, to the analysis performed in the adults from Rosenthal et al.29that included a visual experiment, we wanted to identify the organization of the face selective regions in the ABIDE group and hence, we conducted a second analysis with hemisphere as another within subject factor.We found a main effect of group (F (1,59) = 19.308,p<0.0001, η 2 p = 0.246) and network (F (7,413) = 84.911,p<0.000001, η 2 p =0.590) and an

Table S5A :
Rosenthal et al. 29-Comparison of the connectivity in each hemisphere in each network Significant results are indicated in bold.

Table S5B :
ABIDE -Comparison of the connectivity in each hemisphere in each network

hemispheric dominance in the visual networks
We examined the connectivity in an infant group that was tested in Dinstein et al.38, as well as children, early adolescents, late adolescents, and adult groups from the ABIDE database and crosssectionally examined the changes in the connectivity patterns across age.We first conducted a repeated measures ANOVA with the between factors: group (toddlers from Dinstein et al.38, children from ABIDE, early adolescents from ABIDE, late adolescents from ABIDE and adults from ABIDE), within factors: network (as described in Functional connectivity analysis) and hemisphere.We examined the right hemispheric dominance of the core-face selective regions known from the literature 13 and that we replicated also in the present study (see Supplementary-Comparing visual and resting state experiments in adults from Rosenthal et al.29, Figure3& Supplementarycomparing resting state scans from Rosenthal et al.29

Table S6A :
Toddlers -Comparison of the connectivity in each hemisphere in each network

Table S6B :
Children -Comparison of the connectivity in each hemisphere in each network

Table S6C :
Early adolescents -Comparison of the connectivity in each hemisphere in each network

Table S6D :
Late adolescents -Comparison of the connectivity in each hemisphere in each network Significant results are indicated in bold.

Table S6E :
Adults -Comparison of the connectivity in each hemisphere in each network

Table S8A :
Children -Comparison of the connectivity in each hemisphere in each network Significant results are indicated in bold.

Table S8B :
Adolescents -Comparison of the connectivity in each hemisphere in each network

Table S8C :
Adults -Comparison of the connectivity in each hemisphere in each network Significant results are indicated in bold.